The invention herein relates to the production of human tissue plasminogen activator (tPA) in a transformant host cell culture. More specifically, the invention relates to vectors, cells, and methods of producing tPA in conjunction with expression of the sequences for coding for dihydrofolate reductase (DHFR) protein in such cells.
The production of tPA using recombinant techniques has been disclosed in U.S. Application Ser. No. 397,987, filed July 14, 1982 which is a continuation in part of U.S. Ser. No. 374,860 filed May 5, 1982; the contents of both applications are incorporated herein by reference. These applications describe the construction of plasmids containing the coding sequences for tPA, and describe the activity and utility of tPA so produced.
It has also been found, as set forth in applications U.S. Ser. No. 06/459,152, filed 19 January 1983, now U.S. Pat. No. 4,713,339, and U.S. Ser. No. 06/459,151, filed 19 January 1983, now abandoned. and incorporated herein by reference, that a DNA sequence encoding for a DHFR protein can be utilized as a marker for transfection of a sequence coding for a desired heterologous protein in suitable host cells. The DHFR sequence can also be used as a secondary sequence permitting control of the production of the desired protein. These applications disclose such a use, both of wild type DHFR, and of a mutant DHFR which is resistant to methotrexate.
A problem frequently encountered in the production of polypeptides in a foreign host is the necessity to have some mechanism to regulate, usually to enhance, the production of the desired protein. In the case of tPA, which forms the subject matter of this invention, a secondary coding sequence comprising DHFR which is affected by an externally controlled parameter, such as methotrexate, is utilized to permit control of expression by control of the methotrexate (MTX) concentration.
Methotrexate is a drug which is normally fatal to cells capable of its uptake. However, certain cells are able to grow in the presence of controlled levels of MTX. One of the several mechanisms whereby methotrexate resistance is effected is that whereby amplification of the gene coding for the DHFR coding sequence is stimulated (Schimke, Robert T. et al, Science, 202: 1051 (1978); Biedler, J.L. et al, Cancer Res. 32: 153 (1972); Chang, S.E., et al, Cell, 7: 391 (1976)).
It has further been shown that amplification of the gene for DHFR may further cause amplification of associated sequences which code for other proteins. This appears to be the case when the associated protein is hepatitis B surface antigen (HBsAg) (Christman, J. et al, Proc. Natl. Acad. Sci., 79: 1815 (1982)); the E. coli protein XGPRT (Ringold, Gordon, et al, J. Molec. and Appl. Gen., 1: 165 (1981)); and an endogenous sequence from a DHFR/SV40 plasmid combination (Kaufman, R.F. et al, J. Molec. Biol., 159: 601 (1982)).
Other mechanisms for conferring methotrexate resistance include diminution of the binding affinity of the DHFR protein, so that it is less susceptible to methotrexate (Flintoff, W.F. et al, Somat. Cell Genet., 2: 245 (1976)) but in this instance, amplification appears to occur as well.
Thus it would appear that the genes both for wild type DHFR and for DHFR which is resistant to MTX by virtue of its own decreased binding capacity are amplified by the presence of MTX. Hence, in principle, the invention herein concerns using the impact of DHFR sequence amplification on associated protein coding sequences to provide a control mechanism which permits enhanced expression levels of tPA sequences in the presence of MTX, or by virtue of prior treatment of transformed cells with MTX.
As described in U.S. Ser. No. 397,987, tPA is a fibrinolytic substance which can be recovered from human melanoma cells (EPO Patent Application Publn. No. 0041766). This product has been isolated and characterized [Weiman et al, The Lancet, II (8250): 1018 (1981)]. Its fibrinolytic activity is analogous to that of two commercially available proteins, streptokinase and urokinase, which are indicated for the treatment of acute cardiovascular diseases such as myocardial infarct, stroke, pulmonary embolism, deep vein thrombosis, peripheral arterial occlusion and other venous thromboses. The etiological basis for these diseases is apparently either a partial or total occlusion of a blood vessel by a blood clot. Thus traditional anticoagulant therapy for example, treatment with heparin or coumarin, is not effective as it will merely prevent the formation of further clots, but not result in the dissolution of clots already formed. The fibrinolytic agents, streptokinase, urokinase, and plasminogen activator all operate similarly. They convert the inactive precursor plasminogen into plasmin which is capable of dissolving the fibrin of which these clots are composed. Plasminogen activator has a high affinity for fibrin, and thus preferentially activates plasminogen associated with the fibrin desired to be dissolved. On the other hand, streptokinase and urokinase do not; hence, much of the plasmin formed is formed in circulating blood and is neutralized before it can reach the targeted clot. Furthermore, as these compounds create circulating rather than fibrin bound plasmin, other clotting factor proteins in circulation such as fibrinogen, Factor V, and Factor VIII are also attacked by the activated protein causing a hemorrhagic potential. Furthermore, streptokinase is strongly immunogenic.
Plasminogen activator overcomes the foregoing difficulties by specifically attacking plasminogen already bound to fibrin. The present invention concerns a method of increasing and controlling the production of this valuable protein in recombinant cultures by effecting control on amplification of the sequence for DHFR protein.